Third as well as fourth generation mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architecture are able to support more sophisticated services than simple voice and messaging services offered by previous generations of mobile telecommunication systems. For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as video streaming and video conferencing on mobile communications devices that would previously only have been available via a fixed line data connection. The demand to deploy fourth generation networks is therefore strong and the coverage area of these networks, i.e. geographic locations where access to the networks is possible, is expected to increase rapidly. However, although the coverage and capacity of fourth generation networks is expected significantly exceed those of previous generations of communications networks, there are still limitations on both the network capacity and the geographical areas than can be served by such networks. These limitations may for example be particularly relevant in situations where networks are experiencing high load and high-data rate communications. Although the licensed frequency spectrum available to mobile communications system may increase and therefore allows for capacity to be increased further, future growth in demand may lead to insufficient licensed frequency spectrum being available to provide sufficient capacity to meet demand. As a consequence of this limited spectrum, the use of unlicensed portions of the frequency spectrum has been proposed for LTE systems, where the additional frequency spectrum may be used to supplement the licensed spectrum available and therefore increase the capacity of LTE systems. However, in contrast to licensed portions of the frequency spectrum, unlicensed portions may be used by a broad range of systems which may both cause interference but also be susceptible to interference.